http://knoxpipes.com/new.html Dont know anything about the site but they have a 4 chamber on their first page. Not three though :H
If you have a smoke shop around, or you know someone who lives in a big city tell the person at the headshop (or have your friend tell them) that you are looking for a double perculator water pipe. Some common brands that you might look for are THX, LUX or blue dot(not labeled as that, but you will know if you see one, its got rings of blue dots on it). These three brands are nice but will be pretty expensive. For a double perc LUX i'd say prob around 250-300.
K, maybe nevermind about the THX brand, I dont think thats the name of it, i cant seem to remember what it is, but they'll definately know what you're talking about if you say LUX or ROOR (pretty much the same thing, just LUXs have percs)